Polyolefins are used in various uses. For example, films formed of a straight-chain low-density polyethylene (LLDPE) are superior in various characteristics, for example, these films have excellent heat sealing characteristics, high softness and high toughness, and it also have good water resistance, humidity resistance and chemical resistance and are inexpensive, and are therefore widely utilized.
These polyolefins are typically produced by a liquid phase polymerization method such as solution polymerization and slurry polymerization. Particularly LLDPEs are produced by copolymerizing ethylene with an α-olefin having 4 or more carbon atoms in the presence of a Ziegler catalyst by using a liquid phase polymerization method.
In the meantime, if such a polyolefin is produced by a vapor phase polymerization method, the polymer can be obtained in the form of a particle after polymerization and a step of precipitating particles and a step of separating particles from the polymerization solution become needless and therefore the production process can be simplified. This is the reason why studies concerning the production of polyolefins, particularly, LLDPEs by using a vapor phase polymerization method have been made energetically in recent years.
In the vapor phase polymerization method, polymerization is carried out with fluidizing solid particles consisting of a catalyst and a produced polyolefin by supplying polymerizable monomer gas from the lower part of a reactor to form a fluidized-bed and to thereby run a polymerization reaction and withdrawing the produced polymer continuously or intermittently from the reactor.
In such a vapor phase polymerization method, there is a large problem concerning the removal of polymerization heat. Conventionally, a method is known in which a liquefiable saturated aliphatic hydrocarbon is supplied to a fluidized-bed reactor and the gas discharged from the reactor is compressed and cooled to liquefy a part of the saturated aliphatic hydrocarbon, to thereby circulate the saturated aliphatic hydrocarbon to the reactor in a vapor-liquid mixed state, thereby removing polymerization heat.
However, even if the conventional heat removing method as aforementioned is adopted, it is difficult to remove heat evenly from the fluidized-bed and therefore local heating in the fluidized-bed is easily caused, giving rise to the sheeting and melting polymer problem. When such a sheet or melting polymer is produced, the polymer is deposited on the gas distributing plate disposed on the lower portion of the fluidized-bed and the deposited polymer becomes an obstacle to keep a normal fluidized state and also clogs the holes of the gas distributing plate, which occasionally hinders the normal operation of the reaction system significantly.
In view of this situation, the inventors of the present invention have made earnest studies and as a result, found that the local heating in the fluidized-bed occurs in such a manner as to be concentrated in the peripheral portion of the fluidized-bed reactor and that in order to restrict the local heating, it is effective to introduce a saturated aliphatic hydrocarbon put in a liquid phase state selectively into the peripheral portion of the fluidized-bed to thereby solve the inferior heat-removing in the peripheral portion of the fluidized-bed reactor by heat of vaporization. The present invention has been thus completed.